WO2019219520A1

WO2019219520A1 - Light panel and lighting kit

Info

Publication number: WO2019219520A1
Application number: PCT/EP2019/062016
Authority: WO
Inventors: Huajie LUO; Jing BAO; Zhong Huang; Qin Li; Liang Zhou
Original assignee: Signify Holding B.V.
Priority date: 2018-05-17
Filing date: 2019-05-10
Publication date: 2019-11-21

Abstract

Disclosed is a light panel (10) comprising a light mixing chamber (15) delimited by a reflective plate (30) opposing a light-scattering light exit window (20) and a sidewall arrangement (41, 42) extending between the reflective plate and the light exit window. The sidewall arrangement comprises at least one sidewall (41) carrying a plurality of solid state lighting elements (50) and a lens arrangement (60) arranged over the solid state lighting elements and adapted to redirect a first portion (163) of a luminous output of the solid state lighting elements towards the reflective plate, a second portion (164, 165) of said luminous output towards a central region (21) of the light exit window and a third portion (263) of said luminous output into the light mixing chamber in between the reflective plate and the light exit window, wherein said second portion is larger than any one of the first portion and the third portion. Also disclosed is a lighting kit comprising a plurality of such light panels.

Description

LIGHT PANEL AND LIGHTING KIT

FIELD OF THE INVENTION

The present invention relates to a light panel comprising a light mixing chamber delimited by a reflective plate opposing a light-scattering light exit window and a sidewall arrangement extending between the reflective plate and the light exit window, the sidewall arrangement comprising at least one sidewall carrying a plurality of solid state lighting elements.

The present invention further relates to a lighting kit comprising a plurality of such light panels.

BACKGROUND OF THE INVENTION

Advances in lighting technology such as the introduction of solid state lighting (SSL), e.g. as implemented by light emitting diode (LED)-based lighting modules, has transformed the lighting field. For example, LED-based light panels having surface areas, of up to several square meters (m²) are now available that can transform the lighting experience in enclosed spaces such as large rooms, offices, halls and the like. Such panels, which may be composed of one or more lighting modules, in some application domains are provided as at least part of the ceiling of such enclosed spaces, where they seek to provide substantially homogeneous lighting emanating from parts of the ceiling defined by such panels.

The achievement of such a substantially homogeneous illumination of the light exit window of such a light panel is challenging. For example, in prior art light panels based on fluorescent tubes as the internal light source, the light exit window of such a panel would have a non-uniform, striped appearance caused by the spatially separated fluorescent tubes. With SSL elements such as LEDs, such light sources act as point sources, which when organized in strips facing the light exit window, give a spotted appearance and may even cause glare if such point sources can be directly observed through the light exit window. For this reason, the light exit window regularly consists of a diffusive polymer or fabric (cloth) to obscure the SSL elements from direct view and to improve the homogeneity of the luminous output of the light panel. However, some residual inhomogeneity and direct view of the SSL elements may be difficult to avoid.

Indirectly lit lighting panels in which the SSL elements are arranged on wires that face the reflective plate opposing the light exit window may be used to obscure the SSL elements from direct view, but it is difficult to achieve homogeneous illumination of the light exit window with such an arrangement due to the positioning of such wires in the optical path between the reflective plate and the light exit window, which may lead to dark stripes in the light exit window illuminated by the light reflected by the reflective plate. Also, such a solution is rather costly, for example because of the low yield when welding the SSL elements onto such wires, and therefore less commercially attractive.

For this reason, side-lit light panels are known in which the SSL elements are mounted on a sidewall of the light panel. This effectively obscures the SSL elements from direct view but increases the challenge to achieve a good homogeneity across the light exit window. This may be achieved by using a light guide within the light panel into which the light of the SSL elements is coupled and from which light is extracted by a regular pattern of outcoupling structures that break the total internal reflection of the light travelling within the light guide and cause the light to escape the light guide in the direction of the light exit window of the light panel. Such a solution also has a number of drawbacks. The light guide adds cost and weight to the light panel, whilst it can compromise its optical efficiency due to light losses and limits the total area of the light panel to dimensions of about 600 mm x 600 mm. Also, the regular pattern of outcoupling structures may be replicated to some extend in the luminous distribution across the light exit window, i.e. this luminous distribution may show some residual inhomogeneity.

US 9,562,670 B2 discloses an illumination system comprising a light source and a refractive collimator. The light source is configured to emit a substantially Lambertian light distribution around an axis of symmetry. The refractive collimator is configured to redirect light from the light source so as to at least partially illuminate an illuminating surface, in which at least a part of the illuminating surface is substantially parallel to the axis of symmetry. The refractive collimator comprises a concave entrance window and an at least partially convex exit window for refracting light towards the illuminating surface such that a height of the illumination system may be reduced by virtue of the use of the refractive collimator. However, this solution is less suitable for achieving homogenous illumination of a light exit window of a large area light panel having substantially planar major surfaces, i.e. a substantially planar light exit window and reflective plate.

SUMMARY OF THE INVENTION

The present invention seeks to provide a light panel that can be manufactured in a cost-effective manner, and that can be increased in size without compromising the homogeneity of illumination of its light exit window.

The present invention further seeks to provide a lighting kit comprising a plurality of such light panels.

According to an aspect, there is provided a light panel comprising a light mixing chamber delimited by a reflective plate opposing a light-scattering light exit window and a sidewall arrangement extending between the reflective plate and the light exit window, the sidewall arrangement comprising at least one sidewall carrying a plurality of solid state lighting elements and a lens arrangement arranged over the solid state lighting elements and adapted to redirect a first portion of a luminous output of the solid state lighting elements towards the reflective plate, a second portion of said luminous output towards a central region of the light exit window and a third portion of said luminous output into the light mixing chamber in between the reflective plate and the light exit window, wherein said second portion is larger than any one of the first portion and the third portion. The light-scattering light exit window has a transmittance of normal incident light, i.e. light incident on the light exit window under a 90° angle, within a range of 20% ~ 65% and a reflectance within a range of 75% ~30%, wherein the reflectance is negative correlation to the transmittance.

The present invention is based on the insight that a side-lit light panel having good homogeneity across its light exit window and having good optical yields can be provided in a cost-effective manner by the provision of a lens arrangement over the SSL elements on a sidewall of the light panel that is designed to produce an asymmetric luminous distribution in which the majority of the luminous output of the SSL elements is directed towards the light exit window to reduce light losses and improve the optical efficiency of the light panel. The specific transmittance / reflectance of the light exit window uniforms the light spreading over the whole light exit window although majority of the light projects on the central region of the light exit window.

Preferably, the second portion comprises 48-55% of the luminous output of the solid state lighting elements, whereas the first portion may comprise 25-30% of the luminous output of the solid state lighting elements in order to achieve such homogeneous illumination of the light exit window of the light panel.

In a particular embodiment, opposing edges of neighboring solid state lighting elements are separated from each other by a distance d and a light emitting surface of each solid state lighting element is separated from an opposing light entry surface of the lens arrangement by a height h, and wherein said distance d obeys the relationship < h x tan v in which v is the angle between a mid-point on the opposing light entry surface of the lens arrangement in between the neighboring solid state lighting elements and a mid-point of the light emitting surface of one of said neighboring solid state lighting elements, and v does not exceed 70°. It has been found that when this relationship is obeyed, particularly effective mixing of the respective luminous distributions of the SSL elements on the sidewall is achieved, thereby achieving a particularly homogeneous luminous output with the light panel.

In another embodiment, the sidewall arrangement has a height H, and wherein a distance D between the center of each of the solid state elements and the reflective plate is defined as D = 0.35*H ~ 0.52*H. This further improves the homogeneity of the luminous output of the light panel.

The light panel may comprise a single sidewall carrying SSL elements, in which case the light panel has a width W and the sidewall arrangement comprises a light- reflective further sidewall opposing the sidewall carrying the plurality of solid state lighting elements and wherein a ratio W/H is in a range of 4-10 in order to achieve effective mixing of light in the mixing chamber and achieve a homogeneous luminous output with the light panel as a result. This for instance is a suitable embodiment where the light panel has a width in a range of 300 mm up to 950 mm. For larger area light panels, the sidewall arrangement comprises a further sidewall opposing the sidewall carrying the plurality of solid state lighting elements, said further sidewall carrying a further plurality of solid state lighting elements and a further lens arrangement arranged over the further plurality of solid state lighting elements and adapted to redirect a first portion of a further luminous output of the further plurality of solid state lighting elements towards the reflective plate, a second portion of said further luminous output towards a central region of the light exit window and a third portion of said further luminous output into the light mixing chamber in between the reflective plate and the light exit window, wherein said second portion of the further luminous output is larger than any one of the first portion and the third portion of the further luminous output. This for instance is a suitable embodiment where the light panel has a width in a range from 950 mm up to 1 ,500 mm or more. In such embodiments, the ratio W/H preferably is in a range of 6-20 in order to achieve effective mixing of light in the mixing chamber and achieve a homogeneous luminous output with the light panel as a result.

Preferably, a direction a of a peak intensity of the respective second portions of the luminous output and the further luminous output is a = tan 1 H - D

such that the 0.5 * W

second portion(s) are aimed at the central region of the light exit window in order to homogeneously illuminate the light exit window.

The light panel may further comprise a shielding element on each sidewall carrying a plurality of solid state lighting elements, said shielding element extending on said sidewall in between the solid state lighting elements and the light exit window to suppress illumination hot spots on the light exit window that could deteriorate the homogenous illumination of the light exit window. Such a shielding element for example may be a light-reflective element, a semi-transparent element or a light-absorbent element.

In a preferred embodiment, each lens arrangement comprises a column lens extending over the plurality of solid state lighting elements, which is particularly cost- effective as only a single lens body has be manufactured, e.g. moulded, to cover the plurality of SSL elements on a particular sidewall. Alternatively, each lens arrangement may comprise a plurality of lenses, each lens positioned over one of said solid state lighting elements.

The light-scattering light exit window may comprise a light-scattering polymer or a light-scattering fabric. This ensures a diffused luminous output of the light panel, which may be advantageous in terms of preventing glare and creating an aesthetically pleasing luminous output.

According to another aspect, there is provided a lighting kit comprising a plurality of light panels according to any of the herein described embodiments. In this manner, large area lighting panels may be constructed in a modular manner by combining a plurality of the lighting modules according to one or more embodiments of the present invention, thereby significantly reducing the manufacturing complexity of such large area light panels. A light panel assembly assembled in such a manner may comprise light panels each having a separate body acting light-scattering light exit window such as a woven fabric over the lighting panel. Alternatively, the lighting kit may comprise a body such as a woven fabric that is common to the plurality of light panels such that upon assembly of the light panels the body spans the plurality of light panels.

The light panels may be arranged to be coupled to each other in order to form the light panel assembly. Alternatively, the lighting kit may further comprise a mounting frame for said light panels in which the light panels are to be mounted.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in more detail and by way of non limiting examples with reference to the accompanying drawings, wherein:

FIG. 1 schematically depicts a cross-sectional view of a light panel according to an embodiment;

FIG. 2 schematically depicts a sectional view of an aspect of such a light panel along the elongate direction of a linear array of SSL elements within such a light panel;

FIG. 3 schematically depicts a cross-sectional view of a lens of such a light panel;

FIG. 4 schematically depicts a cross-sectional view of a luminous distribution produced by such a lens; FIG. 5-8 schematically depict various components of this luminous distribution within such a light panel;

FIG. 9 schematically depicts a cross-sectional view of a light panel according to another embodiment;

FIG. 10 schematically depicts a cross-sectional view of a light panel according to yet another embodiment;

FIG. 1 1 schematically depicts a cross-sectional view of a light panel according to yet another embodiment;

FIG. 12 schematically depicts a cross-sectional view of a light panel according to yet another embodiment;

FIG. 13 schematically depicts a perspective view of a light panel according to an example embodiment;

FIG. 14 is a graph of a luminance distribution across a light exit window of a light panel according to an example embodiment; and

FIG. 15 illustrates the relationship of the transmittance and reflectance of the light exit window of the light panel according to an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

FIG. 1 schematically depicts a cross-sectional view of a light panel 10 according to an embodiment of the present invention. The light panel 10 comprises a light exit window 20 having a central region 21 opposing a reflective plate 30 and a sidewall arrangement including opposing sidewalls 41 and 42 extending between the light exit window 20 and the reflective plate 30 such that the sidewall arrangement, the reflective plate 30 and the light exit window 20 combine to delimit a light mixing chamber 15 of the light panel 10 having a height H and a width W. In this example embodiment, the light panel 10 is a rectangular light panel although it should be understood that other shapes may be contemplated. The light panel 10 may be a frameless light panel although embodiments of the present invention are not necessarily limited thereto. The light exit window 20 typically is a light-scattering light exit window producing a Lambertian scattering distribution, which may be formed of a light-scattering material such as a light-scattering polymer or a light-scattering woven fabric or cloth obscuring the internals of the light panel 10. The light-scattering textile may be textured, as will be readily understood by the skilled person. Such a light-scattering material preferably has a light transmittance for light that is incident normal to the plane of the light exit window 20 in a range of 20-65%. This effectively obscures the internals of the light panel 10 and in addition ensures that light generated within the light panel 10 is effectively scattered or diffused by the light-scattering material in order to generate a diffuse luminous output with the light panel 10. In the meantime, the light-scattering material has a reflectance within a range of 75% ~30%, wherein the reflectance is negative correlation to the transmittance. This provides sufficient light to reflect back into the light mixing chamber 15 so that a satisfying homogeneity can be achieved on the light exit window 20. FIG. 15 illustrates the relationship between the transmittance and reflectance.

The reflective panel 30 may be made of any suitable material such as a reflective material, e.g. a metal or metal alloy that may be polished in order to improve its reflectance or a non-reflective material carrying a reflective coating facing the light exit window 20. Any suitable reflective coating may be used for this purpose such as a reflective foil, a reflective paint such as white paint, and so on. The surface of the reflective panel 30 facing the light exit window 20 may be a planar surface although this is not strictly necessary as will be explained in further detail below. The sidewalls of the sidewall arrangement preferably are also light-reflective in order to minimize light losses within the light mixing chamber 15 of the light panel 10. As with the reflective plate 30, the sidewalls of the sidewall arrangement may be made reflective using a reflective material or any suitable reflective coating on a non-reflective material.

In the embodiment depicted in FIG. 1 , the sidewall 41 carries a linear array of SSL elements 50 extending in an elongation direction of the sidewall 41, which SSL elements 50 may be mounted directly onto the sidewall 41 or may be mounted on a carrier (not shown) such as a PCB strip or the like, which carrier is mounted onto the sidewall 41. The SSL elements 50 preferably are white light-emitting LEDs although embodiments of the present invention are not limited thereto.

A lens arrangement 60 is positioned over the linear array of SSL elements 50, as will be explained in further detail below. The lens arrangement 60 preferably consists of a column lens that extends in the elongation direction of the sidewall 41 and is arranged over the linear array of SSL elements 50. Alternatively, the lens arrangement 60 may consist of individual lenses, with each lens covering one of the SSL elements 50 of the linear array. The lens arrangement may be made of any suitable material, e.g. glass or an optical grade polymer such as polycarbonate (PC), poly ethylene terephthalate (PET) or poly (methyl methacrylate) (PMMA) by way of non-limiting examples. The use of an optical grade polymer has the advantage that the lens arrangement may be manufactured in a cost-effective manner compared to glass, e.g. through moulding techniques such as injection moulding.

The lens arrangement 60 is designed to redistribute the luminous output of the SSL elements 50 in such a manner that a substantially homogeneous luminance distribution is achieved across the light exit window 20. In order to achieve such a substantially homogeneous luminance, the linear array of SSL elements 50 is positioned relative to the reflective plate 30 such that a distance D between the center of each SSL elements 50 and the reflective plate 30 is in a range of 0.35*H ~ 0.52*H, in which H is the height of the light mixing chamber 15 as defined as the distance between the reflective panel 30 and the opposing light exit window 20. Lor the sake of completeness, the width W of the light mixing chamber 15 is defined as the distance between the opposing sidewalls 41 and 42, and is the larger width in case of a rectangular light panel 10 that is not square, i.e. having a major width and a minor width, in which case W refers to the major width.

PIG. 2 schematically depicts a portion of the lens arrangement 60 in situ over the SSL elements 50 on the sidewall 41 in which a light entry surface 61 of the lens arrangement 60 has been designed to have a minimum distance or height h to the light emitting surfaces of the SSL elements 50. PIG. 2 is a sectional view taken along the elongate direction of the array of SSL elements 50. Opposing edges of neighboring SSL elements 50 are separated from each other by a distance d and a light emitting surface 51 of each solid state lighting element is separated from an opposing light entry surface 61 of the lens arrangement 60 by a minimum height h.

The distance or spacing d between the neighboring SSL elements 50 obeys the relationship < h x tanv in which v is the angle between a mid-point 62 on the

opposing light entry surface of the lens arrangement in between the neighboring SSL elements 50 which is equidistant to the opposing edges of these SSL elements and a mid point 52 of the light emitting surface of one of said neighboring solid state lighting elements, in which v does not exceed 70°. This relationship is particularly relevant to single colour and (tunable) white light SSL elements 50, and ensures that hot spot formation on the light exit window 20 is effectively avoided when this relationship is obeyed and the SSL elements 50 are regularly spaced within the linear array of SSL elements on the sidewall 41. Such hot spot formation is undesirable as it compromises the homogeneity of the luminance distribution across the light exit window 20.

The operation of the lens arrangement 60 will now be explained in more detail with the aid of FIG. 3, in which a cross-sectional view of an example embodiment of such a lens arrangement 60, i.e. a column lens or an individual lens, is schematically depicted. The lens arrangement 60 is designed such that a peak intensity direction of the light redirected by the lens arrangement 60 is aimed at the central region 21 of the light exit window 20. More specifically, the lens arrangement 60 comprises one or more asymmetric lenses having a light entry surface 61 and an aspherical or multi-faceted central refractive light exit surface 63 flanked by refractive planar surfaces 64 and 65 respectively, which planar surfaces may be angled under different angles to a central axis 66 of the lens arrangement 60 such that light refracted by these surfaces may be aimed as desired. The lens arrangement 60 further comprises surfaces 67 that may act as total internal reflection surfaces.

As shown in FIG. 4, the light exit surfaces of the lens arrangement 60 are designed to refract different portions of the luminous output of the SSL elements 50. For example, in the 2-D view of FIG. 4, the central surface 63 may refract light emitted by the SSL elements 50 under an angle b of 32° to 140° (wherein b is defined as the angle with the axis 250 normal to the light emitting surface of the SSL element 50) , whereas the planar surface 64 is arranged to refract the light emitted by the SSL elements 50 under angles over 140° and the planar surface 65 is arranged to refract the light emitted by the SSL elements 50 under angles below 32°. The lens arrangement 60 is designed such that a first portion 163 of the luminous output of the solid state lighting elements is refracted by a first part of the central region 63 towards the reflective plate 30. The planar surfaces 64 and 65 both aim their respective refracted luminous outputs of the SSL elements 50 at the central region 21 of the light exit window 20, which combined refracted outputs are also referred to as the second portion of the luminous output of the SSL elements 50. A third portion 263 of the luminous output of the SSL elements 50 is refracted by a second part of the central region 63 into the light mixing chamber 15. This is more clearly depicted in FIGs. 5-8 in which the path of these portions through the light mixing chamber 15 is depicted.

The lens arrangement 60 is dimensioned such that the second portion is larger than any of the first and third portions in terms of fraction of the total luminous intensity produced by the SSL elements 50. Preferably, the second portion (i.e. 164+165) contains around 48-52% of this luminous intensity whereas the first portion 163 contains about 25- 30% of this luminous intensity, with the remainder being contained in the third portion 263. It has been found that when these ratios are obeyed together with the herein described design rules for the positioning of the SSL elements 50 and the design rules for the lens arrangement 60, the light exit window 20 may exhibit a luminance in which the minimum luminance L_min is at least 80% of the maximum luminance L_max, i.e. L_min ³ 0.8*L_max, and the luminance L_center in the central region 21 has a ratio to the average luminance L_average across the light exit window 20 of L_Center:L_average > 0.96 such that the central region 21 of the light exit window 20 is effectively illuminated by the lens arrangement 60.

Moreover, by directly illuminating the light exit window 20 with about 48-52% of the total luminous output of the SSL elements 50, an optimal trade-off is achieved between the optical efficiency of the light panel 10 and the homogeneity of the luminance across the light exit window 20. It is noted that it is of course possible to design the lens arrangement 60 such that substantially all its refracted luminous output is directly aimed at the light exit window 20. However, this would require a light exit window having an increased degree of scattering compared to the light exit window 20 of the light panel 10 according to embodiments of the present invention, which would compromise the optical efficiency of the light panel 10, i.e. increase the light losses due to the increased scattering.

In order to achieve these performance figures, the lens arrangement 60 must obey the design rule a = tan 1 H - D

in which the angle a defines the aim of the peak

0.5

intensity of the second portion 164, 165 of the light refracted by the lens arrangement 60 as depicted in FIG. 6. A further constraint is the ratio W:H, which is a function of whether the light panel 10 has a linear array of SSL elements 50 on the sidewall 41 only or on both opposing sidewalls 41, 42.

If only the sidewall 41 carries a linear array of SSL elements 50, the ratio W:H should be in a range of 4-10 to ensure that a sufficiently homogeneous luminance of the light exit window 20 can be achieved. If the ratio becomes larger than 10, the far region of the light exit window 20, i.e. the region proximal to the sidewall 42 may become under-illuminated such that the homogeneity of the luminance across the light exit window 20 can no longer be achieved. Consequently, the width W of such a light panel 10 is typically limited to about 950 mm for a light panel having a height of about 10 mm.

On the other hand, if both opposing sidewalls 41 and 42 carrying a linear array of SSL elements 50 respective lens arrangements 60, the ratio W:H may be in a range of 6~20 to ensure such sufficiently homogeneous luminance of the light exit window 20 such that for such a light panel 10 its total width W may increase to 1 ,250 mm or even 2,000 mm for a light panel 10 having a height of about 10 mm.

Typically, when designing a lens arrangement 60 for such a light panel 10, the first constraint is the height H of the light panel, which dictates what width the light panel 10 can have based on the aforementioned ratios W:H. Upon choosing an appropriate width W, the angle a is used to design the lens arrangement 60 such that the lens arrangement 60 produces the above described light distributions 163, 164, 165 and 263. This may be achieved using any suitable CAD program as will be immediately apparent to the skilled person such that this will not be explained in further detail for the sake of brevity only. The width W may have a tolerance of 66-140% without departing from the teachings of the present application.

FIG. 9 schematically depicts a cross-sectional view of an alternative embodiment of the light panel 10 in which a shielding element 70 is positioned on the sidewall 41 in between the optical arrangement formed by the linear array of SSL elements 50 and the lens arrangement 60 and the light exit window 20. The shielding element 70 prevents the formation of a luminous hotspot on the light exit window 20, for example when the spacing d between neighboring SSL elements 50 in the linear array does not obey the relationship d < h x tan v that has been described in more detail above. Such a shielding element 70 may be a light-reflective element, a semi-transparent or translucent element or a light-absorbent element in order to prevent the formation of such hotspots on the light exit window 20. For example, the shielding element 70 may be made of a semi transparent, diffusive plastic, a non-transparent plastic or a solid metal plate optionally with through-holes across the shielding element, and so on.

FIG. 10 schematically depicts a cross-sectional view of another embodiment of the light panel 10 in which in addition to the sidewall 41 carrying the linear array of SSL elements 50, the opposing sidewall 42 carries a further linear array of solid state lighting elements 50’ and a further lens arrangement 60’ arranged over the solid state lighting elements 50’, such as a further column lens or a further plurality of individual lenses as previously explained. The further lens arrangement 60’ is adapted to redirect a first portion of a further luminous output of the further solid state lighting elements 50’ towards the reflective plate (30), a second portion of this further luminous output towards the central region 21 of the light exit window 20 and a third portion of the further luminous output into the light mixing chamber 15 in between the reflective plate 30 and the light exit window 15. The second portion of the further luminous output is larger than any one of the first portion and the third portion of the further luminous output. In other words, the optical arrangement formed by the further lens arrangement 60’ and the further solid state lighting elements 50’ mirrors the optical performance of the optical arrangement formed by the lens arrangement 60 and the solid state lighting elements 50. As explained above, such a two-sided arrangement of SSL elements 50, 50’ allows for a higher W:H ratio of the light panel 10, such that the larger light panel 10 can be provided (at constant height H) for which substantially homogeneous illumination of its light exit window 20 can be achieved with good optical efficiency.

As can be seen in FIG. 11, for such a two-sided illuminated light panel 10, each of the sidewalls 41 , 42 may carry the aforementioned shielding element 70, 70’ in between the respective linear arrays of SSL elements 50, 50’ and the light exit window 20 to prevent formation of hotspots on either side of the central region 21 of the light exit window 20, i.e. proximal to the respective sidewalls 41, 42.

FIG. 12 schematically depicts a cross-sectional view of a light panel 10 according to another embodiment in which the reflective plate 30 carries a tapered reflective structure 33 in a central region of the reflective plate 30 that narrows towards the light exit window 20. The tapered reflective structure 33 may have any suitable shape such as a pyramidal shape or a conical shape and has the same optical properties as the reflective plate 30. The tapered reflective structure 33 may form an integral part of the reflective plate 30 or may be a separate body attached to the reflective plate 30. The tapered reflective structure typically has a height Db that is less than half the height H of the light mixing chamber 15, i.e. D_b < 0.5*H such that only a minor fraction of the light travelling through the light mixing chamber 15 is redirected by the tapered reflective structure 33. The tapered reflective structure 33 redirects incident light towards the peripheral regions of the light exit window 20, i.e. regions proximal to the opposing sidewalls 41 , 42 to further improve the homogeneity of the luminance across the light exit window 20.

FIG. 13 schematically depicts a perspective view of a light panel 10 according to an example embodiment, in which one of the sidewalls extending between the opposing sidewalls 41, 42 has been removed for the sake of clarity. In this embodiment, both sidewalls 41 , 42 carry an optical arrangement of which the respective lens arrangements 60, 60’ are visible on the opposing sidewalls 41 , 42 extending in between the light exit window 20 and the reflective plate 30.

FIG. 14 is a graph depicting the luminance (Y-axis) across the light exit window 20 (X-axis) of the light panel 10 depicted in FIG. 13. As can be seen from this graph, a highly homogeneous luminance is achieved across the light exit window 20, with the luminance in the central region 21 having an intensity of more than 0.96 times the average luminance across the light exit window 20, i.e. L_Center:L_average > 0.96 and Lmm/L_max >0.85, thus demonstrating that a light panel 10 having the lens arrangement 60 in accordance with embodiments of the present invention can achieve a highly uniform luminance across its light exit window 20.

A plurality of light panels 10 according to embodiments of the present invention may be provided as a lighting kit, in which the light panels 10 may be designed to be coupled to each other, either by fixings and/or by a common mounting frame in which the light panels 10 can be mounted. In such a lighting kit, each light panel 10 may have its own light exit window 20, or alternatively the lighting kit comprises a single light exit window 20, e.g. a single fabric or cloth to be deployed across all the light panels 10 in the lighting kit. Such a lighting kit may be used to form a luminaire in which multiple light panels 10 are combined in a common housing, e.g. a mounting frame or the like, to form the luminaire.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A light panel (10) comprising a light mixing chamber (15) delimited by a reflective plate (30) opposing a light-scattering light exit window (20) and a sidewall arrangement (41, 42) extending between the reflective plate and the light exit window, the sidewall arrangement comprising at least one sidewall (41) carrying a plurality of solid state lighting elements (50) and a lens arrangement (60) arranged over the solid state lighting elements and adapted to redirect a first portion (163) of a luminous output of the solid state lighting elements towards the reflective plate, a second portion (164, 165) of said luminous output towards a central region (21) of the light exit window and a third portion (263) of said luminous output into the light mixing chamber in between the reflective plate and the light exit window, wherein said second portion is larger than any one of the first portion and the third portion;

wherein the light-scattering light exit window (20) has a transmittance of normal incident light within a range of 20% ~ 65% and a reflectance within a range of 75% ~30%, wherein the reflectance is negative correlation to the transmittance.

2. The light panel (10) of claim 1 , wherein the second portion (164, 165) comprises 48-55% of the luminous output of the solid state lighting elements (50).

3. The light panel (10) of claim 1 or 2, wherein the first portion (163) comprises 25- 30% of the luminous output of the solid state lighting elements (50).

4. The light panel (10) of any of claims 1-3, wherein opposing edges of neighboring solid state lighting elements (50) are separated from each other by a distance d and a light emitting surface (51) of each solid state lighting element is separated from an opposing light entry surface (61) of the lens arrangement (60) by a height h, and wherein said distance d obeys the relationship—d < h x tan v in which v is the angle between a mid point (62) on the opposing light entry surface of the lens arrangement in between the neighboring solid state lighting elements and a mid-point (52) of the light emitting surface of one of said neighboring solid state lighting elements, and v does not exceed 70°.

5. The light panel (10) of any of claims 1 -4, wherein the sidewall arrangement (41, 42) has a height H, and wherein a distance D between the center of each of the solid state elements (50) and the reflective plate (30) is defined as D = 0.35*H ~ 0.52*H.

6. The light panel (10) of claim 5, wherein the light panel has a width W and the sidewall arrangement (41 , 42) comprises a light-reflective further sidewall (42) opposing the sidewall (41) carrying the plurality of solid state lighting elements and wherein a ratio W/H is in a range of 4- 10.

7. The light panel (10) of claim 5, wherein the sidewall arrangement (41, 42) comprises a further sidewall (42) opposing the sidewall (41) carrying the plurality of solid state lighting elements (50), said further sidewall carrying a further plurality of solid state lighting elements (50’) and a further lens arrangement (60’) arranged over the further plurality of solid state lighting elements and adapted to redirect a first portion of a further luminous output of the further plurality of solid state lighting elements towards the reflective plate (30), a second portion of said further luminous output towards a central region (21) of the light exit window (20) and a third portion of said further luminous output into the light mixing chamber (15) in between the reflective plate and the light exit window, wherein said second portion of the further luminous output is larger than any one of the first portion and the third portion of the further luminous output.

8. The light panel (10) of claim 7, wherein the ratio W/H is in a range of 6-20.

9. The light panel (10) of claim 7 or 8, wherein a direction a of a peak intensity of the respective second portions of the luminous output and the further luminous output is

_! H - D

a = tan -

0.5 * W

10. The light panel (10) of any of claims 1-9, further comprising a shielding element (70, 70’) on each sidewall (41, 42) carrying a plurality of solid state lighting elements (50, 50’), said shielding element extending on said sidewall in between the solid state lighting elements and the light exit window (20).

11. The light panel (10) of claim 10, wherein the shielding element (70, 70’) is a light-reflective element, a semi-transparent element or a light-absorbent element.

12. The lighting panel (10) of any of claims 1 -11 , wherein each lens arrangement (60,

60’) comprises a column lens extending over the plurality of solid state lighting elements (50, 50’) or a plurality of lenses, each lens positioned over one of said solid state lighting elements.

13. The lighting panel (10) of any of claims 1 -12, wherein the light-scattering light exit window (20) comprises a light-scattering polymer or a light-scattering fabric.

14. A lighting kit comprising a plurality of light panels (10) according to any of claims 1 -13.

15. The lighting kit of claim 14, further comprising a mounting frame for said light panels (10).